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An object of the present invention is to provide a technique capable of,
in an image projection apparatus and a laser beam projection apparatus
that reduce speckle noise by means of high-frequency superimposition,
reducing a deterioration in the light emission intensity of a
semiconductor laser while suppressing EMI. To achieve the object, the
image projection apparatus includes a plurality of semiconductor lasers
for emitting laser beams of different wavelengths in accordance with
drive currents supplied thereto, a laser-drive circuit for generating the
drive currents by superimposing a plurality of high-frequency signals on
image signals of a plurality of color components, respectively, and a
deflection section for deflecting each of the laser beams, wherein the
plurality of high-frequency signals have different fundamental
frequencies from one another.

Inventors:

Uchino; Hiroshi; (Kyoto, JP)

Serial No.:

266769

Series Code:

13

Filed:

March 15, 2010

PCT Filed:

March 15, 2010

PCT NO:

PCT/JP2010/054289

371 Date:

October 27, 2011

Current U.S. Class:

345/690; 353/31

Class at Publication:

345/690; 353/31

International Class:

G09G 5/02 20060101 G09G005/02; G03B 21/14 20060101 G03B021/14

Foreign Application Data

Date

Code

Application Number

Apr 27, 2009

JP

2009107472

Claims

1. An image display apparatus for projecting an image on a projection
surface comprising: a laser-drive circuit for superimposing each of a
plurality of high-frequency signals on corresponding one of a plurality
of image signals for different color components to generate drive
currents; a plurality of semiconductor lasers for emitting laser beams of
different wavelengths in accordance with drive currents, respectively;
and a deflection section for deflecting each of said laser beams onto
said projection surface, wherein said plurality of high-frequency signals
have different fundamental frequencies from one another.

2. The image projection apparatus according to claim 1, wherein for said
plurality of semiconductor lasers, a semiconductor-laser drive period of
each of said drive currents is set so as to monotonically increase
sequentially from one of said plurality of semiconductor lasers in which
the sum of an oscillation delay time period and a relaxation oscillation
duration is shorter.

3. The image projection apparatus according to claim 1, wherein for said
plurality of semiconductor lasers, a semiconductor-laser drive period of
each of said drive currents is set so as to be equal to the sum of an
oscillation delay time period and a relaxation oscillation duration.

4. The image projection apparatus according to claim 1, wherein for said
plurality of semiconductor lasers, a semiconductor-laser drive period of
each of said drive currents is set so as to monotonically increase
sequentially from one of said plurality of semiconductor lasers that
emits the laser beam having a wavelength to which human's luminosity
factor is higher.

5. The image projection apparatus according to claim 1, wherein said
plurality of high-frequency signals have different harmonic frequencies
from one another in a range up to a predetermined-order harmonic.

6. A laser beam projection apparatus for projecting a laser beam,
comprising: a laser-drive circuit for superimposing each of a plurality
of high-frequency signals on corresponding one of a plurality of image
signals for different color components to generate drive currents; and a
plurality of semiconductor lasers for emitting laser beams of different
wavelengths in accordance with drive currents, respectively wherein said
plurality of high-frequency signals have different fundamental
frequencies from one another.

7. A laser beam projection apparatus for projecting a laser beam,
comprising: a plurality of semiconductor lasers for emitting laser beams
of different wavelengths in accordance with drive currents supplied
thereto; and a laser-drive circuit for generating each of said drive
currents that serve as cycle signals, such that a semiconductor-laser
drive period of each of said drive current can be equal to the sum of an
oscillation delay time period and a relaxation oscillation duration
corresponding to each of said plurality of semiconductor laser, wherein
each of said drive currents is generated based on a high-frequency signal
and an image signal corresponding to each of said plurality of
semiconductor lasers, said drive currents have different cycles from one
another.

Description

TECHNICAL FIELD

[0001] The present invention relates to an image projection apparatus that
projects an image using a laser beam.

BACKGROUND ART

[0002] A display using a laser as a light source can reproduce natural
colors with a wider color range, as compared with using a discharge lamp
as the light source. Particularly, a laser-beam scan type projector
requires no projection optical system, and therefore can be downsized as
compared with a method of using a projection optical system to project an
illumination light modulated by a two-dimensional spatial modulation
device such as a liquid crystal panel or a DMD (digital micromirror
device).

[0003] The laser-beam scan type projector can also reduce power
consumption, because it changes the amount of light emission from a light
source in accordance with the luminance of a pixel to be displayed, such
as stopping the light emission from the light source in a case of
displaying a black pixel. In this manner, the laser-beam scan type
projector has excellent features as a display device, and is attracting
attention as a next-generation high-quality display.

[0004] On the other hand, a laser display has a problem specific thereto
that due to a high coherence of the laser beam, non-uniformity of the
luminance on a viewing surface occurs to cause eye flickering called
speckle noise (hereinafter also referred to as "speckle"), so that a
person viewing the displayed image suffers discomfort and eye strain.
Thus, a reduction in the speckle noise is one of technical problems
involved in practical use of the laser display.

[0005] There are generally two methods for reducing speckle noise. In
first one of the methods, a generated speckle pattern is changed over
time, and a plurality of patterns being overlapped on one another are
shown by utilizing an afterimage effect of human eyes, to thereby average
the non-uniformity of the luminance and thus reduce the speckle noise.

[0006] According to Patent Document 3 that adopts the first method, in a
rear-projection type television using a laser beam as a light source, a
screen is rotated in parallel with a viewing surface, to thereby change a
speckle pattern over time and thus reduce speckle noise.

[0007] In second one of the methods for reducing speckle noise, a
plurality of polarization states are overlapped on one another or a
plurality of wavelengths are overlapped on one another, to thereby reduce
a coherence of a light emitted from a light source and thus reduce
speckle noise.

[0008] According to Patent Document 1 and Patent Document 2 that adopt the
second method, a single-mode semiconductor laser (also referred to as
"LD", "laser diode", or the like) is used as a laser light source, and
the LD is used in a multi-mode by using a relaxation oscillation that
occurs at a time when a drive current of the LD having a high-frequency
signal superimposed thereon exceeds an oscillation threshold current so
that the LD starts light emission. In this method, a light-emission
wavelength interval is increased by causing the LD to emit a light in the
multi-mode, to thereby reduce a coherence of a laser beam emitted from
the LD and thus reduce speckle noise.

[0009] FIG. 1 shows an example of transient response characteristics in a
case where the LD is driven by a square-wave current. In FIG. 1, the
horizontal axis represents a time axis t, and the vertical axis
represents the number of photons S. As shown in FIG. 1, an ideal response
is a square wave Ri in which the number of photons S rises simultaneously
with a drive current waveform. Actually, however, the LD starts light
emission after elapse of an oscillation delay time period td, and a
waveform Rr of the number of photons S repeats a vibration called a
relaxation oscillation, and then converges to its steady-state value.
Here, a time period tr for which the relaxation oscillation is sustained
is referred to as a relaxation oscillation duration.

[0010] The oscillation delay time period td normally changes within a
range of approximately 0 to 5 nsec due to a bias current of the LD prior
to the rise of the drive current of the LD, and becomes shorter as the
bias current is closer to the oscillation threshold current. Since a
shorter oscillation delay time period td is more preferable, the bias
current is normally brought closer to the oscillation threshold current
and the LD is driven such that the oscillation delay time period td can
fall within a range of 0 to 1 nsec. Normally, the relaxation oscillation
duration tr is approximately 2 to 3 nsec. While the relaxation
oscillation is occurring, the LD emits a light in the multi-mode, and
when the relaxation oscillation converges, the LD emits a light in the
single-mode.

[0011] Thus, it is possible that the single-mode LD repeatedly emits a
light in the multi-mode, by a method in which the relaxation oscillation
exhibited in the waveform Rr is repeatedly generated by repeatedly
performing: using a drive current of a continuous square wave to cause
the LD being in a light-emission stop state to emit a light; and then
setting the drive current lower than the oscillation threshold current of
the LD before the relaxation oscillation converges, to thereby stop the
light emission from the LD.

[0012] FIG. 2 shows an example of the transient response characteristics
in a case where the LD is driven by a continuous square-wave current. In
FIG. 2, the vertical axis and the horizontal axis are set in the same
manner as in FIG. 1.

[0013] Each of the square waves Ri shown in FIG. 2 indicates an ideal
response of the number of photons S with respect to the drive current of
the continuous square wave. The rise and fall of each square wave Ri are
coincident with the rise and fall of the continuous square-wave current
generated by superimposing a high-frequency signal, respectively.
Therefore, in FIG. 2, the t1 represents a time period (also referred to
as "semiconductor-laser drive period" or "pulse width") during one cycle
of the drive current of the LD having the high-frequency signal
superimposed thereon, in which the current value of the drive current is
equal to or more than the current value of the oscillation threshold
current of the LD, while the t0 represents a time period (also referred
to as "semiconductor-laser non-drive period") during one cycle of the
drive current of the LD having the high-frequency signal superimposed
thereon, in which the current value of the drive current is less than the
current value of the oscillation threshold current of the LD.

[0014] This driving method is called a high-frequency superimposition
method, and widely used as a method for suppressing mode hopping noise
caused by a return light in pickup of CD/DVD or the like. This is
effective as a method for reducing speckle noise in a laser display, too,
because a coherence of a projected laser beam is reduced due to the
multi-mode.

[0018] However, the method adopted in the Patent Document 3 requires a
mechanism for mechanically driving the screen, which causes problems of
increased power consumption, occurrence of noise, a lowered reliability,
an increased cost due to an increase in the number of component parts,
and the like. In an apparatus such as a small-size portable projector
(categorized as a so-called pocket projector in recent years) using no
special screen, an idea of moving a screen is not possible in the first
place.

[0019] In the high-frequency superimposition method adopted by the Patent
Document 1 and the Patent Document 2, it is necessary that the drive
current of the LD is lowered to a value equal to or less than the
oscillation threshold current in each cycle of the high-frequency signal
in order to generate the relaxation oscillation.

[0020] Although a shorter length of the semiconductor-laser non-drive
period t0 of the drive current of the LD is preferable because it can
suppress a reduction in the light intensity, the drive current of the LD
in a laser projector is approximately a few hundred mA and therefore it
is difficult, in terms of circuit design, to extremely shorten the length
of the semiconductor-laser non-drive period t0. In actual designing,
normally, a time period of about 1 nsec is required.

[0021] In this manner, performing the high-frequency superimposition
creates a time period in which the light emission from the LD is stopped,
thus reducing an average amount of light emission from the LD. The
reduction in the amount of light emission leads to a reduction in the
maximum luminance of an image that is displayed using a single LD, and
therefore is a problem in a laser projector requiring a high light
intensity.

[0022] Here, a ratio r of a time period in which the LD emits a light is
calculated by the expression (1) using the semiconductor-laser drive
period t1 and the semiconductor-laser non-drive period t0 of the drive
current of the LD, and the oscillation delay time period td of the LD.

[0023] [Math. 1]

r=(t1-td)/(t1+t0) (1)

[0024] In order to suppress a reduction in the amount of light emission,
the frequency of the high-frequency signal superimposed by the
high-frequency superimposition is lowered to thereby increase the length
of the semiconductor-laser drive period t1 of the drive current, thus
increasing the ratio r of the time period in which the LD emits a light.
However, in a case where the frequency of the high-frequency signal
superimposed by the high-frequency superimposition is lowered so that the
semiconductor-laser drive period t1 of the drive current becomes longer
than the sum of the oscillation delay time period td and the relaxation
oscillation duration tr and thus exceeds the relaxation oscillation
duration tr to cause the LD to emit a light, the light emission from the
LD is the single-mode and therefore the speckle-noise reduction effect is
deteriorated. In this manner, in the high-frequency superimposition
method, a trade-off relationship is established between the speckle-noise
reduction effect and the amount of light emission from the LD.

[0025] Moreover, in the laser projector, since a high light intensity is
required, the drive current of the LD is large, too. Thus, the
high-frequency superimposition method involves a problem that occurrence
of EMI (electromagnetic interference) due to an electromagnetic wave
caused by use of a high-frequency drive current makes it impossible to
satisfy the noise standard when mounting a laser beam projection
apparatus on an equipment, and a problem that the size and the cost of
the laser beam projection apparatus are increased by a circuit for
generating the high-frequency drive current and a shield structure
necessary for an anti-EMI product.

[0026] Particularly, in an apparatus including a plurality of LDs, such as
a color projector, if respective LDs of a laser beam projection apparatus
including a plurality of LDs are driven by a drive current having the
same frequency as in Patent Document 1, the peak of EMI becomes large at
a fundamental frequency thereof and each harmonic frequency thereof. This
consequently causes a problem that, when the laser beam projection
apparatus is mounted on an image projection apparatus or the like, the
noise standard cannot be satisfied or alternatively a shield structure or
the like serving as an anti-EMI member is required, which increases the
cost.

[0027] Therefore, an object of the present invention is to provide an
image projection apparatus and a laser beam projection apparatus that
reduce speckle noise by using high-frequency superimposition and that can
suppress EMI while reducing a deterioration in the light emission
intensity of a semiconductor laser.

Means for Solving the Problems

[0028] To solve the above-described problems, an image projection
apparatus according to a first aspect includes: a laser-drive circuit for
superimposing each of a plurality of high-frequency signals on
corresponding one of a plurality of image signals for different color
components to generate drive currents; a plurality of semiconductor
lasers for emitting laser beams of different wavelengths in accordance
with drive currents respectively; and a deflection section for deflecting
each of the laser beams onto the projection surface. In the image
projection apparatus, the plurality of high-frequency signals have
different fundamental frequencies from one another.

[0029] An image projection apparatus according to a second aspect is the
image projection apparatus according to the first aspect, in which for
the plurality of semiconductor lasers, a semiconductor-laser drive period
of each of the drive currents is set so as to monotonically increase
sequentially from ones of the plurality of semiconductor lasers in which
the sum of an oscillation delay time period and a relaxation oscillation
duration is shorter.

[0030] An image projection apparatus according to a third aspect is the
image display projection apparatus according to the first aspect, in
which for the plurality of semiconductor lasers, a semiconductor-laser
drive period of each of the drive currents is set so as to be equal to
the sum of an oscillation delay time period and a relaxation oscillation
duration.

[0031] An image projection apparatus according to a fourth aspect is the
image projection apparatus according to the first aspect, in which for
the plurality of semiconductor lasers, a semiconductor-laser drive period
of each of the drive currents is set so as to monotonically increase
sequentially from one of the plurality of semiconductor lasers that emits
the laser beam having a wavelength to which human's luminosity factor is
higher.

[0032] An image projection apparatus according to a fifth aspect is the
image projection apparatus according to the first aspect, in which the
plurality of high-frequency signals have different harmonic frequencies
from one another in a range up to a predetermined-order harmonic.

[0033] A laser beam projection apparatus according to a sixth aspect is a
laser beam projection apparatus for projecting a laser beam, including: a
laser-drive circuit for superimposing each of a plurality of
high-frequency signals on corresponding one of a plurality of image
signals for different color components to generate drive currents; and a
plurality of semiconductor lasers for emitting laser beams of different
wavelengths in accordance with the drive currents, respectively. In the
laser beam projection apparatus, the plurality of high-frequency signals
have different fundamental frequencies from one another.

[0034] A laser beam projection apparatus according to a seventh aspect is
a laser beam projection apparatus for projecting a laser beam, including:
a plurality of semiconductor lasers for emitting laser beams of different
wavelengths in accordance with drive currents supplied thereto; and a
laser-drive circuit for generating each of the drive currents that serve
as cycle signals, such that a semiconductor-laser drive period of each of
the drive current can be equal to the sum of an oscillation delay time
period and a relaxation oscillation duration corresponding to each of the
plurality of semiconductor laser. In the laser beam projection apparatus,
each of the drive currents is generated based on a high-frequency signal
and an image signal corresponding to each of the plurality of
semiconductor lasers, and the drive currents have different cycles from
one another.

Effects of the Invention

[0035] In the image projection apparatus according to any of the first to
fifth aspects and in the laser beam projection apparatus according to the
sixth aspect, the plurality of high-frequency signals to be superimposed
on the image signals of the plurality of color components for generating
the drive currents of the plurality of semiconductor lasers by the
laser-drive circuit have different fundamental frequencies. As a result,
spectrums of EMIs caused based on the fundamental frequencies of the
high-frequency signals when the plurality of semiconductor lasers are
driven can be dispersed to lower a peak value of the EMI, so that the EMI
can be efficiently suppressed.

[0036] In the image projection apparatus according to the second aspect,
for the plurality of semiconductor lasers, the semiconductor-laser drive
period of each of the drive currents of the plurality of semiconductor
lasers is set so as to monotonically increase sequentially from one of
the plurality of semiconductor lasers in which the sum of the oscillation
delay time period and the relaxation oscillation duration is shorter.
This can efficiently suppress speckle noise while reducing a
deterioration in the light emission intensity.

[0037] In the image projection apparatus according to the fourth aspect,
for the plurality of semiconductor lasers, the semiconductor-laser drive
period of each of the drive currents of the plurality of semiconductor
lasers is set so as to monotonically increase sequentially from one of
the plurality of semiconductor lasers that emits the laser beam having a
wavelength to which human's luminosity factor is higher. This can
efficiently suppress visual speckle noise while reducing a deterioration
in the light emission intensity.

[0038] In the laser beam projection apparatus according to the seventh
aspect, the semiconductor-laser drive period of each of the drive
currents that serve as cycle signals to be supplied to the plurality of
semiconductor lasers is equal to the sum of the oscillation delay time
period and the relaxation oscillation duration corresponding to each of
the plurality of semiconductor lasers, and the drive currents have
different cycles. This can efficiently suppress speckle noise while
dispersing spectrums of EMIs caused by the respective drive currents to
thereby lower a peak value of the EMI.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1 A diagram showing an example of response characteristics of
an LD responsive to an input of a square wave.

[0040] FIG. 2 A diagram showing an example of response characteristics of
the LD responsive to an input of a continuous square wave.

[0041] FIG. 3 A block diagram showing an outline of an exemplary
functional configuration of an image projection apparatus according to an
embodiment.

[0042] FIG. 4 A block diagram showing an outline of an exemplary
functional configuration of a light-source drive section according to the
embodiment.

[0043] FIG. 5 A block diagram showing an outline of a functional
configuration of an LD drive circuit according to the embodiment.

[0044] FIG. 6 A diagram partially showing an example of a signal waveform
in a laser-drive circuit according to the embodiment.

[0045] FIG. 7 A diagram showing an outline of an exemplary functional
configuration of an optical mechanism section according to the
embodiment.

[0046] FIG. 8 A diagram showing an example of response characteristics of
the LD responsive to an input of a square wave.

[0047] FIG. 9 A diagram showing an example of response characteristics of
the LD responsive to an input of a square wave.

[0048] FIG. 10 A diagram showing an example of response characteristics of
the LD responsive to an input of a square wave.

[0049] FIG. 11 A diagram for explaining the human's spectral luminous
efficiency with respect to various wavelengths of lights.

EMBODIMENT FOR CARRYING OUT THE INVENTION

Outline Configuration of Image Projection Apparatus

[0050] FIG. 3 is a block diagram showing an exemplary functional
configuration of an image projection apparatus 100 according to an
embodiment of the present invention.

[0051] The image projection apparatus 100 is an apparatus for projecting a
moving image to a screen SC serving as a projection surface, and mainly
includes an input-image processing section 110, a drive control section
120, and an optical mechanism section 130. The image projection apparatus
100 is equivalent to the "image display projection apparatus" of the
invention of the present application.

[0052] The input-image processing section 110 includes an image input
circuit 111, an image processing circuit 112, and a frame memory 124. The
image input circuit 111 receives an image signal inputted from an input
device IM, and outputs it to the image processing circuit 112. The image
processing circuit 112 appropriately performs image processing such as
digitalization on the image signal supplied from the image input circuit
111, and outputs a resulting signal to an image output circuit 121 of the
drive control section 120 (which will be described later). The image
signal outputted to the image output circuit 121 is written into the
frame memory 124 by the image output circuit 121.

[0053] Here, examples of the input device IM include a personal computer
(PC), and examples of the image signal include a general NTSC signal.
Examples of the image processing performed by the image processing
circuit 112 include general .gamma.-correction processing and processing
of changing the order of pixel values in a case where a change of the
order of scanning pixels is required.

[0054] The drive control section 120 includes the image output circuit
121, a deflection control circuit 122, a light-source drive section 123,
and a sensor-output processing circuit 125.

[0055] The image output circuit 121 reads out the image signal from the
frame memory 124, and in response to a horizontal synchronization signal
and a vertical synchronization signal of the image signal, outputs to the
deflection control circuit 122 a signal (also referred to as "deflection
control signal") for controlling a timing of driving a two-dimensional
deflection section 132, and also outputs to the light-source drive
section 123 a signal (also referred to as "pixel data signal")
corresponding each color component (red, green, and blue) of a pixel
value of the image signal, a signal (also referred to as "frequency
setting signal") for setting a frequency (fundamental frequency) of a
fundamental wave of a high-frequency signal used for high-frequency
superimposition to generate a drive current of an LD of each color that
emits a light of each color component, and a signal (also referred to as
"bias-current setting signal") for setting a bias current of the LD of
each color. The pixel data signal, the frequency setting signal, and the
bias-current setting signal will be further described later.

[0056] The image output circuit 121 receives an output signal (which will
be described later) from the sensor-output processing circuit 125, and in
accordance with the strength of the output signal, adjusts the strength
of the pixel data signal to be outputted to the light-source drive
section 123.

[0057] The deflection control circuit 122 supplies, to the two-dimensional
deflection section 132, a drive signal having a potential in accordance
with the deflection control signal supplied from the image output circuit
121.

[0058] The light-source drive section 123 outputs, to each of a plurality
semiconductor lasers (which will be described later) provided in a laser
light source section 133, a drive current in accordance with a pixel data
signal supplied from the image output circuit 121, such that the
semiconductor laser can emit a light whose color and luminance are in
accordance with a tone corresponding to the pixel data signal supplied
from the image output circuit 121. This output operation is performed in
response to the horizontal synchronization signal and the vertical
synchronization signal of the image signal.

[0059] The sensor-output processing circuit 125 performs necessary
processes such as an amplification process on the output signal outputted
from a light-receiving element 14 (FIG. 7) of the laser light source
section 133, and outputs a resulting signal to the image output circuit
121.

[0060] The input-image processing section 110 and the drive control
section 120 may be functionally implemented by a CPU reading and
executing a predetermined program, or alternatively may be configured as
a special electronic circuit.

[0062] The laser light source section 133 has a plurality of semiconductor
lasers that emit laser beams of the respective colors of red, green, and
blue. The laser light source section 133 outputs to the two-dimensional
deflection section 132 a laser beam whose color and luminance are in
accordance with a tone corresponding to the pixel data signal supplied
from the image output circuit 121, and also outputs to the sensor-output
processing circuit 125 an electrical signal in accordance with the
intensity of each laser beam.

[0063] The two-dimensional deflection section 132 has a part (reflection
part) that reflects a luminous flux of a laser beam emitted from the
laser light source section 133. The reflection part rotates around two
substantially perpendicular axes, to thereby deflect the luminous flux
emitted from the laser light source section 133 in a two-dimensionally
reflecting manner, so that the luminous flux is guided onto the screen SC
serving as the projection surface, thus projecting a moving image on the
screen SC.

[0064] Here, the light-source drive section 123 and the laser light source
section 133 correspond to a "laser beam projection apparatus" of the
present invention. In FIG. 3, a laser beam traveling from the laser light
source section 133 through the two-dimensional deflection section 132 to
the screen SC is expressed by the arrow of thick broken line. A specific
configuration of the optical mechanism section 130 including the
two-dimensional deflection section 132 and the laser light source section
133 will be further described later.

[0065] In this manner, the image projection apparatus 100, the input
device IM, and the screen SC configure an image projection system that
outputs an image data supplied from the input device IM such that the
image data can be visible on the screen SC.

[0066] <Outline Configuration of Light-Source Drive Section>

[0067] FIG. 4 is a block diagram showing an outline of an exemplary
functional configuration of the light-source drive section 123 according
to the embodiment of the present invention.

[0068] The light-source drive section 123 is a circuit for supplying a
drive current to each of the semiconductor lasers 1R, 1G, and 1B (FIG. 7)
of the laser light source section 133, and mainly includes a red-LD drive
circuit 123R, a green-LD drive circuit 123G, and a blue-LD drive circuit
123B. The light-source drive section 123 corresponds to a "laser-drive
circuit" of the present invention.

[0069] The image output circuit 121 supplies, to the red-LD drive circuit
123R, a pixel data signal 11R corresponding to the red color component of
the pixel value of the image signal, and a frequency setting signal 12R
and a bias-current setting signal 13R corresponding to the semiconductor
laser 1R.

[0070] Likewise, the image output circuit 121 supplies, to the green-LD
drive circuit 123G, a pixel data signal 11G corresponding to the green
color component of the pixel value of the image signal, and a frequency
setting signal 12G and a bias-current setting signal 13G corresponding to
the semiconductor laser 1G.

[0071] Thus, the image output circuit 121 supplies, to the blue-LD drive
circuit 123B, a pixel data signal 11B corresponding to the blue color
component of the pixel value of the image signal, and a frequency setting
signal 12B and a bias-current setting signal 13B corresponding to the
semiconductor laser 1B.

[0072] Based on the pixel data signal, the frequency setting signal, and
the bias-current setting signal thus supplied, the red-LD drive circuit
123R, the green-LD drive circuit 123G, and the blue-LD drive circuit 123B
generates drive currents IdR, IdG, and IdB for driving the semiconductor
lasers 1R, 1G, and 1B, respectively, and supply them to the semiconductor
laser. Each of the drive currents IdR, IdG, and IdB corresponds to "each
drive current" of the present invention.

[0073] The frequency setting signals 12R, 12G, and 12B, and the
bias-current setting signals 13R, 13G, and 13B are adjustable by means
of, for example, changing the setting of a DIP switch of predetermined
bit provided in the image output circuit 121.

[0074] <Outline Configuration of Drive Circuit of Each Light Source>

[0075] The red-LD drive circuit 123R, the green-LD drive circuit 123G, and
the blue-LD drive circuit 123B employ the same circuit configuration.
Here, an outline configuration of each LD drive circuit will be described
by taking the red-LD drive circuit 123R as an example.

[0076] FIG. 5 is a block diagram showing an outline of an exemplary
functional configuration of the red-LD drive circuit 123R, and FIG. 6 is
a diagram partially showing an example of a signal waveform in the red-LD
drive circuit 123R.

[0077] As shown in FIG. 5, the red-LD drive circuit 123R mainly includes
D/A converters (also referred to as "DAC") 4, 5, and 6, an analog switch
(also referred to as "Analog SW") 7, a voltage control oscillator (also
referred to as "VCO") 8, an LD driver 9, and a bias current source 10.

[0079] By the DAC 4, the pixel data signal 11R is converted into an analog
voltage V1 (FIG. 5, FIG. 6), and then inputted to the analog switch 7.

[0080] Also in the green-LD drive circuit 123G and the blue-LD drive
circuit 123B, analog voltages V6 and V7 (not shown) corresponding to the
analog voltage V1 are generated, respectively. Here, the analog voltages
V1, V6, and V7 correspond to "a plurality of color components" of the
present invention.

[0081] By the DAC 5, the frequency setting signal 12R corresponding to the
semiconductor laser 1R is converted into an analog voltage V2, and then
supplied to the VCO 8. In the VCO 8, a high-frequency signal V3 (FIG. 5,
FIG. 6) having a fundamental frequency in accordance with the analog
voltage V2 is generated and supplied to the analog switch 7. That is, the
high-frequency signal V3 is a high-frequency signal associated with the
semiconductor laser 1R.

[0082] Also in the green-LD drive circuit 123G and the blue-LD drive
circuit 123B, high-frequency signals V8 and V9 (not shown) corresponding
to the high-frequency signal V3 that are high-frequency signals
associated with the semiconductor lasers 1G and 1B, respectively, are
generated. Here, each of the high-frequency signals V3, V8, and V9
correspond to "a high-frequency signal" of the present invention.

[0083] The fundamental frequencies of the high-frequency signals V3, V8,
and V9 can be adjusted by adjusting the frequency setting signals 12R,
12G, and 12B.

[0084] The analog switch 7 uses the high-frequency signal V3 as a control
input signal, and if the high-frequency signal V3 is higher than a
predetermined threshold voltage TH1 (FIG. 6) of a control input, outputs
the analog voltage V1 while if the high-frequency signal V3 is lower than
the threshold voltage TH1, outputs a ground ("GND")--level signal V0.

[0085] In other words, a signal V4 (FIG. 5, FIG. 6) obtained by chopping
the analog voltage V1 based on the pixel data signal 11R with the
fundamental frequency of the high-frequency signal V3 supplied from the
VCO 8 is outputted from the analog switch 7, and supplied to the LD
driver 9. Accordingly, as shown in FIG. 6, the fundamental frequency of
the signal V4 has the same value as that of the fundamental frequency of
the high-frequency signal V3, and the signal V4 is delayed behind the
analog voltage signal V1 and the high-frequency signal V3 due to a delay
in the circuit, for example.

[0086] Here, the threshold voltage TH1 can be adjusted by, for example,
adjusting a resistance (not shown) that determines the value of the
threshold voltage TH1, which allows adjustment of a duty cycle between an
image portion of the signal V4, that is, a signal corresponding to the
analog voltage V1, and a signal corresponding to the GND-level signal V0.

[0087] The LD driver 9 converts the signal V4 into a current I4 in
accordance with a signal level thereof. Further, a bias current IbR
(which will be described later) is added to the current I4, thus
generating the drive current IdR for driving the semiconductor laser 1R.
That is, the drive current IdR is a drive current that is generated by
superimposing the high-frequency signal V3 associated with the
semiconductor laser 1R on the analog voltage V1 and that has the same
fundamental frequency as that of the high-frequency signal V3.

[0088] In the same manner, the drive currents IdG and IdB (FIG. 4) are
drive currents that are generated by superimposing the high-frequency
signals V8 and V9 associated with the semiconductor lasers 1G and 1B on
the analog voltages V6 and V7, respectively, and that have the same
fundamental frequencies as those of the high-frequency signals V8 and V9,
respectively.

[0089] By the DAC 6, the bias-current setting signal 13R is converted into
an analog voltage signal V5, and then inputted to the bias current source
10. The bias current source 10 outputs the bias current IbR in accordance
with the inputted analog voltage signal V5. As mentioned above, the bias
current IbR is added to the current I4.

[0090] An oscillation delay time period td (FIG. 8) of the semiconductor
laser 1R is changed by the bias current IbR. As the bias current IbR
becomes lower than the oscillation threshold current of the semiconductor
laser 1R, the oscillation delay time period td is increased to reduce an
average amount of light emission from the semiconductor laser 1R.

[0091] On the other hand, when the bias current IbR exceeds the
oscillation threshold current of the semiconductor laser 1R, the
semiconductor laser 1R emits a light even at a black level on the image,
which deteriorates an image contrast. Therefore, it is desirable to
control the bias current IbR to a value as close to the oscillation
threshold current as possible but within a range not exceeding the
oscillation threshold current.

[0092] <Outline Configuration of Optical Mechanism Section>

[0093] FIG. 7 is a diagram showing an example of an outline configuration
of the optical mechanism section 130. As shown in FIG. 7, the optical
mechanism section 130 includes the two-dimensional deflection section 132
and the laser light source section 133. In FIG. 7, coordinate axes are
shown for the indication of directions.

[0094] The two-dimensional deflection section 132 corresponds to
"deflection means" of the present invention, and is formed of, for
example, a so-called MEMS (Micro Electro Mechanical Systems) mirror which
is obtained by performing microfabrication on a silicon chip. The
two-dimensional deflection section 132 includes a movable part that has a
plurality of piezoelectric elements, a torsion bar, and the like, and a
deflection scanning mirror 132a that reflects a luminous flux emitted
from the laser light source section 133.

[0095] A potential drive signal supplied from the deflection control
circuit 122 in accordance with the deflection control signal causes the
plurality of piezoelectric elements to expand or contract as appropriate,
thus deforming the movable part, so that the deflection scanning mirror
132a rotates around the two substantially perpendicular axes (an a-axis
substantially parallel to the X axis in FIG. 7 and a b-axis substantially
parallel to the Y-axis in FIG. 7). As a result of this rotation, the
luminous flux emitted from the laser light source section 133 is
deflected in a two-dimensionally reflecting manner. In this
specification, the wording "deflecting in a two-dimensional direction" is
used to express a state where the reflection part rotates around the two
axes to thereby change a traveling direction of the luminous flux with
respect to the vertical direction and the horizontal direction
independently of each other, in other words, a state where the luminous
flux is deflected in the vertical direction while being also deflected in
the horizontal direction.

[0096] Here, a deflection drive signal for achieving a low-speed rotation
of the deflection scanning mirror 132a around the a-axis and a deflection
drive signal for achieving a high-speed rotation of the deflection
scanning mirror 132a around the b-axis by using resonance driving thereof
are superimposed on each other and then applied to the plurality of
piezoelectric elements. This allows the deflection scanning mirror 132a
to simultaneously perform the low-speed rotation around the a-axis and
the high-speed rotation around the b-axis using the resonance driving.
Thus, by deflecting the laser beam in two different directions,
horizontal scanning and vertical scanning with the laser beam can be
simultaneously performed on the screen SC. Two-dimensional scanning in
which the horizontal scanning and the vertical scanning are
simultaneously performed using a single element is preferable in view of
reducing the number of component parts of the two-dimensional deflection
section 132, and also preferable in view of reducing the manufacturing
cost and reducing operations required for adjustment of elements.

[0097] The laser light source section 133 includes the semiconductor
lasers 1R, 1G, and 1B that emit laser beams, and a lens (collimator lens)
that converts the laser beam emitted from the semiconductor laser into a
substantially parallel luminous flux.

[0098] Here, the laser light source section 133 includes a pair of the
semiconductor laser 1R that produces a red (R) laser beam 15R and a
collimating lens that converts the laser beam 15R into a substantially
parallel luminous flux, a pair of the semiconductor laser 1G that
produces a green (G) laser beam 15G and a collimator lens 2G that
converts the laser beam 15G into a substantially parallel luminous flux,
and a pair of the semiconductor laser 1B that produces a blue (B) laser
beam 15B and a collimator lens 2B that converts the laser beam 15B into a
substantially parallel luminous flux. Here, each of the semiconductor
lasers 1R, 1G, and 1B corresponds to a "semiconductor laser" of the
present invention.

[0099] For example, semiconductor lasers that emit laser beams having
wavelengths 630 nm, 532 nm, and 445 nm are adopted as the semiconductor
lasers 1R, 1G, and 1B, respectively. In accordance with the drive
currents IdR, IdG, and IdB supplied from the light-source drive section
123 at a timing responsive to the horizontal synchronization signal and
the vertical synchronization signal of the image signal, each of the
semiconductor lasers 1R, 1G, and 1B produces and emits a laser beam whose
luminance corresponds to each color component (red, green, blue) of the
pixel value of the image signal. For example, a semiconductor laser that
emits a laser beam in an infrared region may be adopted, and a laser beam
emitted from the semiconductor laser may be subjected to wavelength
conversion using an SHG (second harmonic generation) element so that a
visible light is emitted.

[0100] The laser beams 15R, 15G, and 15B converted into the substantially
parallel luminous fluxes are, as luminous fluxes substantially in
parallel with the Z-axis of FIG. 7, inputted to a combining section 3.

[0101] The combining section 3 includes a dichroic mirror 3R for
reflecting the red laser beam 15R while passing lights having the other
wavelengths, a dichroic mirror 3G for reflecting the green laser beam 15G
while passing lights having the other wavelengths, and a dichroic mirror
3B for reflecting the blue laser beam 15B while passing lights having the
other wavelengths. The dichroic mirror 3B may be an ordinary mirror that
reflects any of the laser beams. Instead of each of the dichroic mirrors,
a dichroic prism may be adopted, and instead of the ordinary mirror, a
ordinary prism that reflects any of the laser beams may be adopted.

[0102] The position and angle of each semiconductor laser and the position
and angle of each dichroic mirror of the combining section 3 are
appropriately adjusted. The laser beams 15R, 15G, and 15B inputted
substantially in parallel with the Z-axis to the combining section 3 are
incident on the dichroic mirrors 3R, 3G, and 3B, respectively, and
combined into a single luminous flux 15a substantially parallel with the
Y-axis due to the function of the dichroic mirrors, and then outputted
from the combining section 3, to be inputted to a branching element 8.

[0103] For example, the branching element 8 can be implemented by forming
an uncoated portion in a part (for example, a central part) of a
reflective mirror coating so that a luminous flux inputted to the
uncoated portion can pass therethrough while a luminous flux inputted to
the remaining portion can be reflected, or by applying a dielectric
multiplayer coating over an entire area of a substrate made of, for
example, glass where a luminous flux is inputted to thereby form a
semi-transmissive mirror (also referred to as a "leakage mirror") having
both of transmission characteristics and reflection characteristics so
that the whole of the inputted luminous flux can pass through with a
predetermined transmittance while the whole of the inputted luminous flux
can be reflected with a predetermined reflectance. The transmittance
value and the reflectance value of the branching element 8 such as a
semi-transmissive mirror are appropriately set to be 95% and 5%,
respectively, for example.

[0104] In the former branching element that passes therethrough a luminous
flux inputted to the uncoated portion while reflecting a luminous flux
inputted to the remaining portion, the intensity distribution of a
transmitted light and a reflected light is damaged in a cross-section
perpendicular to the luminous flux, and the directivity of the
transmitted light and the reflected light is deteriorated due to
diffraction. Although this deterioration may not damage the usability of
the apparatus, it is rather desirable to adopt the latter leakage mirror
as the branching element 8.

[0105] The laser beam 15a inputted to the branching element 8 is branched
into a laser beam 15b and a laser beam 15c. The laser beam 15b passes
through the branching element 8 toward the +Y direction substantially in
parallel with the Y-axis. The laser beam 15c is reflected by the
branching element 8 toward the -Z direction substantially in parallel
with the Z-axis.

[0106] The laser beam 15b is inputted to the two-dimensional deflection
section 132, and deflected in a two-dimensional direction by the
deflection scanning mirror 132a, and then projected onto the screen SC.
The laser beam 15c is inputted to the light-receiving element 14 that
outputs an electrical signal in accordance with the intensity of the
laser beam 15c, then converted into an electrical signal, and outputted
to the sensor-output processing circuit 125.

[0107] Examples of the light-receiving element 14 include a photodiode of
current-output type, and a photodetector sensor of current-output type or
voltage-output type. The sensor-output processing circuit 125 of the
image projection apparatus 130 is provided with a current-voltage
converter, an amplifier, or the like, in accordance with a sensor adopted
as the light-receiving element 14.

[0108] <Frequency Selection Method>

[0109] Next, a description will be given of response characteristics of
the semiconductor laser at a time of high-frequency superimposition, and
a method for selecting a frequency of the high-frequency signal suitable
for the response characteristics. FIGS. 8, 9, and 10 are diagrams showing
examples of response characteristics, responsive to an input of a square
wave, of the semiconductor lasers 1R, 1G, and 1B (FIG. 7) that emit
lights of the wavelengths of red color, green color, and blue color,
respectively.

[0110] Example in which a high-frequency signal having the same
fundamental frequency is superimposed:

[0111] As shown in FIGS. 8, 9, and 10, the sum of an oscillation delay
time period td and a relaxation oscillation duration tr is smallest in
the semiconductor laser 1G that emits a green light, and is 2.4 nsec.

[0112] Accordingly, in a case where the high-frequency superimposition is
performed on the three semiconductor lasers 1R, 1G, and 1B using the
three high-frequency signals V3, V8, and V9 having the same fundamental
frequency to thereby obtain the drive currents of the respective
semiconductor lasers, it is necessary to prevent any of the three
semiconductor lasers from oscillating in a single-mode and to set the
semiconductor-laser drive period t1 (FIG. 2) of each of the drive
currents IdR, IdG, and IdB of the semiconductor lasers to be 2.4 nsec or
less based on the semiconductor laser 1G, in order to maintain a
speckle-noise reduction effect.

[0113] When a semiconductor-laser non-drive period t0 (FIG. 2) of each of
the drive currents IdR, IdG, and IdB is defined as 1 nsec, a fundamental
frequency f of each of the high-frequency signals V3, V8, and V9 is
calculated based on the expression (2). In this case, therefore, the
high-frequency signals V3, V8, and V9 having the same fundamental
frequency of 294 MHz or more are required.

[0114] [Math. 2]

f=1/(t1+t0) (2)

[0115] Here, if it is assumed that the semiconductor-laser non-drive
period t0 of the drive current of each semiconductor laser is 1 nsec and
any of the fundamental frequencies f of the high-frequency signals V3,
V8, and V9 corresponding to the drive currents of the respective
semiconductor lasers is 294 MHz, a ratio r of a time period in which each
of the semiconductor lasers 1R, 1G, and 1B emits a light is obtained as
the value indicated in Table 1, based on the expression (1). In Table 1,
the upper row of "Red LD", "Green LD", "Blue LD", and "Average" indicate
the semiconductor lasers 1R, 1G, 1B, and the average of the semiconductor
lasers, respectively, and the lower row of numerical values indicate the
ratios of the time periods of the respective semiconductor lasers emit
lights, and the average of the ratios of the time periods in which the
respective semiconductor lasers emit lights.

[0116] In a case where the semiconductor lasers are driven by using the
drive currents IdR, IdG, and IdB of the semiconductor laser that are
generated by superimposing the high-frequency signals V3, V8, and V9
having the same fundamental frequency on the analog voltages V1, V6, and
V7, respectively, the peak of EMI becomes large at frequencies of the
fundamental wave each harmonic wave of the drive current, as mentioned
above.

[0117] Therefore, in an example described below, the frequency of the
high-frequency signal to be used for the high-frequency superimposition
for generating the drive current of each LD is selected in accordance
with the response characteristics of the LD such that suppression of the
EMI and suppression of a deterioration in the light emission intensity
can be obtained while maintaining the speckle reduction effect in the
semiconductor lasers 1R, 1G, and 1B.

[0119] Firstly, an example of setting will be described in which a
condition (also referred to as "condition 1") is satisfied that the drive
currents IdR, IdG, and IdB have different fundamental frequencies, that
is, the high-frequency signals V3, V8, and V9 used for high-frequency
superimposition have different fundamental frequencies, and in addition a
condition (also referred to as "condition 2") is satisfied that the
semiconductor-laser drive period of the drive current monotonically
increases sequentially from one of the semiconductor lasers 1R, 1G, and
1B in which the sum of the oscillation delay time period and the
relaxation oscillation duration is shorter.

[0120] In an exemplary case where the drive currents IdR, IdG, and IdB
satisfy the conditions 1 and 2, as indicated by the response
characteristics of the respective LDs shown in FIGS. 8 to 10, while the
sum of the oscillation delay time period td and the relaxation
oscillation duration tr differs among the semiconductor lasers 1R, 1G,
and 1B, the frequency setting signals 12R, 12G, and 12B supplied from the
image output circuit 121 are adjusted to thereby adjust the fundamental
frequencies of the high-frequency signals V3, V8, and V9, so that the
semiconductor-laser drive periods t1 of the drive currents IdR, IdG, and
IdB can be set equal to the sum of the oscillation delay time period td
and the relaxation oscillation duration tr of the corresponding
semiconductor lasers, respectively. Thereby, in the response
characteristics of the respective LDs shown in FIGS. 8 to 10, the
semiconductor-laser drive period's t1 of the drive currents IdR, IdG, and
IdB are 3.6 nsec, 2.4 nsec, and 2.8 nsec, respectively.

[0121] Accordingly, if the semiconductor-laser non-drive period t0 of each
of the drive currents IdR, IdG, and IdB is set to 1 nsec similarly to the
above-described example, the fundamental frequencies of the drive
currents of the semiconductor lasers 1R, 1G, and 1B, that is, the
fundamental frequencies f of the high-frequency signals V3, V8, and V9,
are 217 MHz, 294 MHz, and 263 MHz, respectively, based on the expression
(2).

[0122] In this case, the ratio r of a time period in which each of the
semiconductor lasers 1R, 1G, and 1B emits a light is obtained as the
value indicated in Table 2, based on the expression (1). The denotations
of Table 2 are the same as those of Table 1.

[0123] Table 3 shows the fundamental frequency and each harmonic frequency
(unit: MHz) of each of the high-frequency signals V3, V8, and V9 used for
the high-frequency superimposition on the analog voltages V1, V6, and V7
for generating the drive currents IdR, IdG, and IdB in this case. In
Table 3, the items "Fundamental Wave" to "Seventh Harmonic Wave"
represent a fundamental wave through a seventh harmonic of a
high-frequency signal corresponding to each semiconductor laser, and the
items "Red LD", "Green LD", and "Blue LD" represent the semiconductor
lasers 1R, 1G, and 1B, respectively. In the table, the frequencies
appearing in italic type represent frequencies being higher than the
highest frequency among the fifth harmonic frequencies of the
high-frequency signal used for high-frequency superimposition with
respect to each of the semiconductor lasers 1R, 1G, and 1B.

[0124] Firstly, from Table 3, it can be found that spectrums of EMIs
caused by the drive currents IdR, IdG, and IdB of the LDs are dispersed
without overlapping each other at the fundamental frequencies of the
respective drive currents.

[0125] That is, as shown in Table 3, the fundamental frequencies of the
high-frequency signals V3, V8, and V9 paired with the semiconductor
lasers 1R, 1G, and 1B, respectively, satisfy the condition 1. In other
words, the fundamental frequencies of the respective high-frequency
signals are set to be different from one another. Thereby, in an EMI
caused by the fundamental wave and each harmonic wave of each of the
drive currents IdR, IdG, and IdB that are generated by superimposition of
the high-frequency signals V3, V8, and V9 on the analog voltages V1, V6,
and V7, respectively; a spectrum of the EMI caused by the fundamental
wave, which may produce the maximum EMI peak, is dispersed without
overlap. Therefore, a peak value of the EMI caused by the fundamental
waves that occur when the plurality of semiconductor lasers 1R, 1G, and
1B are driven is lowered as compared with, for example, a case where all
of the high-frequency signals used for high-frequency superimposition
have the same fundamental frequency. Thus, the EMI can be suppressed
efficiently.

[0126] Thus, from the viewpoint of suppression of EMI, it is more
desirable that high-frequency signals used for high-frequency
superimposition for generating the drive currents of the plurality of LDs
have different fundamental frequencies, than that all of the
high-frequency signals have the same fundamental frequency.

[0127] Table 1 and Table 2 reveal the fact that, in a case of Table 2, the
ratios of the time periods in which the semiconductor lasers 1R and 1B
emit lights increases so that a deterioration in the average amount of
total light emission from each semiconductor laser is reduced as compared
with the case of Table 1 where all of the high-frequency signals V3, V8,
and V9 to be superimposed on the analog voltages V1, V6, and V7 for
generating the drive currents of the semiconductor lasers 1R, 1G, and 1B
have the same fundamental frequency.

[0128] That is, the semiconductor lasers 1R, 1G, and 1B are set such that
the semiconductor-laser drive period t1 of the drive current can
monotonically increase sequentially from one of the semiconductor lasers
1R, 1G, and 1B in which the sum of the oscillation delay time period td
and the relaxation oscillation duration tr is shorter. Thereby, while
efficiently suppressing speckle noise in each semiconductor laser in
accordance with differences in the response characteristics among the
respective semiconductor lasers responsive to the drive currents, a
deterioration in the light emission intensity of each LD can be reduced
as shown in Table 2. In addition, a deterioration in the average amount
of total light emission of each semiconductor laser can also be reduced.

[0129] Moreover, Table 3 reveals that the spectrums of the EMIs caused by
the drive currents IdR, IdG, and IdB of the LDs are dispersed without
overlapping each other at least up to the fifth harmonic wave.

[0130] Here, in the EMI standard, a measurement frequency range is defined
as 1 GHz or lower according the VCCI standard, and as a fifth harmonic
wave of the maximum frequency or lower according to the FCC standard.
Therefore, in the example shown in Table 3, no frequency overlap occurs
within the range defined by the VCCI standard and the FCC standard.

[0131] Merely when the high-frequency signals V3, V8, and V9 have
different fundamental frequency values, an advantageous effect can be
obtained. However, when the frequencies of the high-frequency signals V3,
V8, and V9 are set so as not to overlap each other in a range from the
fundamental wave to a predetermined-order harmonic wave, for example, up
to the fifth harmonic wave, not only the spectrums of EMIs caused by the
fundamental frequencies of the high-frequency signals V3, V8, and V9 but
also the spectrums of EMIs caused by the harmonic frequencies up to a
predetermined-order harmonic frequency are dispersed. This can further
lower the peak value of the EMI caused when the plurality of
semiconductor lasers 1R, 1G, and 1B are driven, to allow efficient
suppression of the EMI.

[0133] Next, an example will be shown in which the frequency of the
high-frequency signal used for high-frequency superimposition is selected
in accordance with the response characteristics of each LD such that a
greater speckle reduction effect can be exhibited in an LD in which
speckle noise tends to be prominent.

[0134] FIG. 11 is a diagram for explaining the human's spectral luminous
efficiency (also referred to as "luminosity function") with respect to
lights of various wavelengths. FIG. 11 reveals that, in a case of
adopting LDs that emit laser beams having wavelengths of 645 nm, 532 nm,
and 445 nm as a red light, a green light, and a blue light, respectively,
the luminosity factors for the red light and the blue light are lower
than the luminosity factor for the green light. The green light
corresponding to a high luminosity factor causes the most prominent
speckle noise to the human eye, and therefore it is important to reduce
the speckle noise caused by the LD that emits a green laser beam. On the
other hand, in the blue laser beam having the lowest luminosity, less
speckle noise is caused. Therefore, putting priority on the suppression
of the deterioration in the light intensity, the semiconductor-laser
drive period t1 (FIG. 2) of the drive current of the semiconductor laser
that emits the blue light can be set to be longer than the sum of the
oscillation delay time period td and the relaxation oscillation duration
tr.

[0135] Next, an example will be shown in which the frequencies of the
high-frequency signals V3, V8, and V9 to be superimposed on the analog
voltages V1, V6, and V7 for generating the drive currents IdR, IdG, and
IdB of the respective semiconductor lasers are selected in consideration
of the human's luminosity factor with respect to the semiconductor lasers
1R, 1G, and 1B that have the transient response characteristics shown in
FIGS. 8 to 10 and that emit laser beams of wavelength of red-color,
green-color, and the blue-color.

[0136] Here, an example of setting will be described in which the
condition 1 mentioned in Example 1 is satisfied, in other words, the
high-frequency signals V3, V8, and V9 used for high-frequency
superimposition have different fundamental frequencies while a condition
(also referred to as "condition 3") is satisfied that the
semiconductor-laser drive period t1 of the drive current monotonically
increases sequentially from one of the semiconductor lasers 1R, 1G, and
1B that emits a laser beam having a wavelength to which the human's
luminosity factor is higher.

[0137] The frequency setting signals 12R, 12G, and 12B supplied from the
image output circuit 121 are adjusted to thereby adjust the fundamental
frequencies of the high-frequency signals V3, V8, and V9, so that the
semiconductor-laser drive period t1 of each drive currents can be set
equal to the sum of the oscillation delay time period td and the
relaxation oscillation duration tr of each of the semiconductor lasers
1R, 1G, and 1B shown in the response characteristics of FIGS. 8 to 10.
Thereby, the semiconductor-laser drive periods t1 of the drive currents
of the semiconductor lasers 1R, 1G, and 1B are 3.6 nsec, 2.4 nsec, and
2.8 nsec, respectively. Here, in order that the drive currents IdR, IdG,
and IdB can satisfy the conditions 1 and 3, the semiconductor-laser drive
periods t1 of the drive currents of the semiconductor lasers 1R and 1G
causing more prominent speckle noise are set to be slightly shorter for
priority on the speckle-noise reduction effect, while the
semiconductor-laser drive period t1 of the semiconductor laser 1B causing
less prominent speckle is set to be longer for priority on ensuring of
the light intensity. In this case, for example, if time periods of 3.3
nsec, 2.2 nsec, and 4.0 nsec are adopted as the semiconductor-laser drive
periods t1 of the drive currents of the semiconductor lasers 1R, 1G, and
1B, respectively, while a time period of 1 nsec is adopted as the
semiconductor-laser non-drive period t0 of each drive current; the
fundamental frequencies of the drive currents of the semiconductor lasers
1R, 1G, and 1B, that is, the fundamental frequencies f of the
high-frequency signals V3, V8, and V9, are 233 MHz, 313 MHz, and 200 MHz,
respectively, based on the expression (2).

[0138] In this case, the ratio r of a time period in which each of the
semiconductor lasers 1R, 1G, and 1B emits a light is obtained as the
value indicated in Table 4, based on the expression (1). The denotations
of Table 4 are the same as those of Table 1.

[0139] Table 5 shows the fundamental frequency and each harmonic frequency
(unit: MHz) of each of the high-frequency signals V3, V8, and V9 used for
the high-frequency superimposition on the analog voltages V1, V6, and V7
for generating the drive currents IdR, IdG, and IdB in this case. The
denotations of Table 5 are the same as those of Table 3.

[0140] Firstly, from Table 5, it can be found that spectrums of EMIs
caused by the drive currents IdR, IdG, and IdB of the LDs are dispersed
without overlapping each other at the fundamental frequencies of the
respective drive currents.

[0141] This is, similarly to the result shown in Table 3 in Example 1,
because of the following. That is, the fundamental frequencies of the
high-frequency signals V3, V8, and V9 paired with the semiconductor
lasers 1R, 1G, and 1B, respectively, satisfy the condition 1, and in
other words, they are set to be different from one another. Thereby, in
an EMI caused by the fundamental wave and each harmonic wave of each of
the drive currents IdR, IdG, and IdB that are generated by
superimposition of the high-frequency signals V3, V8, and V9,
respectively; a spectrum of the EMI caused by the fundamental wave, which
may produce the maximum EMI peak, is dispersed without overlap.
Therefore, a peak value of the EMI caused by the fundamental waves that
occur when the plurality of semiconductor lasers 1R, 1G, and 1B are
driven is lowered as compared with a case where all of the high-frequency
signals used for high-frequency superimposition have the same fundamental
frequency. Thus, the EMI can be suppressed efficiently.

[0142] Thus, from the viewpoint of suppression of EMI, it is more
desirable that high-frequency signals used for high-frequency
superimposition have different fundamental frequencies, than that all of
the high-frequency signals have the same fundamental frequency.

[0143] As shown in Table 2 and Table 4, regarding the semiconductor lasers
1R and 1G, the ratio of the light-emission time period is shorter in the
result shown in Table 4 than in the result shown in Table 2 because of
priority on the speckle-noise reduction effect, while regarding the
semiconductor laser 1B, the ratio of the light-emission time period is
performed increases in the result shown in Table 4 as compared with the
result shown in Table 2 because of priority on increase in the ratio of
the light-emission time period.

[0144] From Table 1 and Table 4, it can be found that an average
deterioration in the light emission intensity of the semiconductor lasers
can be reduced in Table 4 showing the result of setting the fundamental
frequencies of the high-frequency signals used for high-frequency
superimposition for the respective semiconductor lasers such that visual
speckle noise can be reduced in consideration of the human's luminosity
factor to the wavelength of the laser beam emitted from each
semiconductor laser, as compared with Table 1 showing the result of
setting the fundamental frequencies of the high-frequency signals V3, V8,
and V9 to be superimposed on the analog voltages V1, V6, and V7 for
generating the drive currents of the semiconductor lasers 1R, 1G, and 1B
such that all of them can have the same fundamental frequency.

[0145] That is, the setting is made such that the semiconductor-laser
drive period t1 of the drive current can monotonically increase
sequentially from one of the semiconductor lasers 1R, 1G, and 1B that
emits a laser beam having a wavelength to which the human's luminosity
factor is higher. Thereby, in accordance with the human's luminosity
factor to the wavelength of the laser beam emitted from each
semiconductor laser, the visual speckle noise of each semiconductor laser
can be suppressed in an efficient manner while a deterioration in an
average amount of light emission of each semiconductor laser can also be
reduced as shown in Table 4.

[0146] Moreover, Table 5 reveals that the spectrums of the EMIs caused by
the drive currents IdR, IdG, and IdB of the LDs are dispersed without
overlapping each other at least up to the fifth harmonic wave.

[0147] Therefore, in the example shown in Table 5, similarly to the result
shown in Table 3 of Example 1, no frequency overlap occurs within the
range defined by the VCCI standard and the FCC standard.

[0148] Merely when the high-frequency signals V3, V8, and V9 have
different fundamental frequency values, an advantageous effect can be
obtained. However, when the frequencies of the high-frequency signals V3,
V8, and V9 are set so as not to overlap each other in a range from the
fundamental wave to a predetermined-order harmonic wave, for example, up
to the fifth harmonic wave, not only the spectrums of EMIs caused by the
fundamental frequencies of the high-frequency signals V3, V8, and V9 but
also the spectrums of EMIs caused by the harmonic frequencies up to a
predetermined-order harmonic frequency are dispersed. This can further
lower the peak value of the EMI caused when the plurality of
semiconductor lasers 1R, 1G, and 1B are driven, to allow efficient
suppression of the EMI.

[0149] Although there is no overlap at the harmonic wave of the drive
signal of each LD in Example 1 and Example 2, overlap may occur in the
fundamental wave or the harmonic wave depending on the transient response
characteristics of an LD used. In such a case, if the EMI arises a
problem at the frequency where the overlap occurs, frequency overlap can
be avoided by shifting a drive frequency for any of LDs causing the
frequency overlap. Here, which of the LDs should be the target of
changing the frequency of the high-frequency signal corresponding to the
drive current may be appropriately set in consideration of the
speckle-noise reduction effect and a deterioration in the light emission
intensity.

[0150] Although in Example 1, the frequency of the high-frequency signal
to be superimposed in each LD is selected such that the
semiconductor-laser drive period t1 can be equal to the sum of the
oscillation delay time period td and the relaxation oscillation duration
tr, the frequency of the high-frequency signal for each LD may be
adjusted in accordance with a demand concerning the speckle-noise
reduction effect and the light emission intensity.

[0151] In the same manner, in Example, the frequency of the high-frequency
signal for each LD may be adjusted in accordance with a demand concerning
the speckle-noise reduction effect and the light emission intensity.